1 . Field of the Invention
This invention relates generally to solar cells. More particularly, this invention relates to solar cells having a heterojunction. Even more particularly, this invention relates to solar cells having a GaAs absorber layer.
2. Description of the Prior Art
The current state-of-the-art GaAs solar cells having a GaAs absorber layer and a GaAs emitter layer are limited in their efficiency (less than about 24.8%) by the quality of the GaAs emitter layer. The high recombination velocity at the GaAs surface (even when passivated, e.g., with AlGaAs) reduces the V.sub.oc, J.sub.sc, and fill factor of these devices
One obstacle to increasing the conversion efficiency of GaAs solar cells has been the quality of the GaAs emitter. The blue portion of the incident light spectrum is absorbed mainly in the emitter. Hence, the effective lifetime of photogenerated minority carriers in the emitter must be long enough to assure their complete collection. However, there are several loss mechanisms in the GaAs that limit this collection efficiency. First, most of the current in the device must flow laterally in the emitter. Therefore, its sheet resistance must be small. For a given emitter thickness, one is forced to use a high doping density, which in turn reduces the minority-carrier lifetime. Second, defects at the front surface also reduce the minority-carrier lifetime.
U.S. Pat. No. 4,017,332 (James) discloses a cell for converting received light energy to electrical energy, comprising: four layers of differing types of semiconductive materials stacked to form three opposite conductivity junctions, wherein the outer two "active" junctions are formed of confronting layers with matched lattice constants to provide a plurality of energy converters, and the center connective junction is formed by two confronting intermediate layers which have purposely mismatched lattice constants to provide a lattice defect site surrounding the center junction.
U.S. Pat. No. 4,591,654 (Yamaguchi et al.) discloses an InP solar cell comprising a p-type InP single crystal substrate having a defined carrier concentration and an n-type InP layer containing a dopant of at least one element selected from groups VIA including S and Se disposed on the substrate.
The Conference Record, 13th IEEE Photovoltaic Specialists Conference, June, 1978, pages 886-891, discloses a (AlGa)As-GaAs-Ge dual junction cell made monolithically depositing GaAs and Ge layers epitaxially on the back side of a state-of-the-art AlGaAs/GaAs solar cell to provide two photovoltaicallyactive junctions, in the GaAs and in the Ge, respectively.
IEEE Transactions on Electron Devices, Vol. ED-27, No. 4, April 1980, pages 822-831, discloses materials for fabrication of cells including the use of Ga.sub.x In.sub.1-x P (x being less than 0.35 ) layers to increase the efficiency of multi junction solar cells beyond that of single-junction cells.
However, these references only disclose multi- or single-junction devices that utilize GaInP, either as a buffer layer (as in the Yamaguchi et al. reference) or as a homojunction material in a multijunction device.
Even though the current state-of-the-art has to a significant extent resolved many of the mechanical problems encountered by providing workable contacts to the various layers in the stack or stacking cells in a relatively efficient manner (i.e., through the use of n-on-p GaAs solar cells) these cells have been found to be limited in conversion efficiency due to the quality of the n-GaAs emitter layer. Further, in the n-on-p GaAs solar cells (and also in the p-on-n type), the high recombination velocity at the GaAs surface, even when passivated with AlGaAs, reduces the V.sub.oc,J.sub.sc, and the fill factor of these devices.